The present invention relates to an improved servo control system, and in particular to one particularly suited for use in controlling a linear actuator motor in a magnetic disc file system.
The two major functions of the transducer head-positioning servo mechanism in a magnetic disc file are track-accessing and track-following. The track-accessing function provides minimum time movement of a recording head from its existing track, which is a circular recording band at a specified disc radius, to a different track specified by the file controller. The track-following function maintains the position of the recording heads exactly over the center of a given track with minimum displacement error in the presence of disturbances. Examples of systems for performing the latter function are found in U.S. Pat. Nos. 3,534,344 and 3,691,543.
The present invention is related to the track-accessing function provided by the head-positioning servo mechanism. The track-following servo is useful only over distances of approximately plus or minus one-half track, thus is incapable of controlling a transducer head motion over distances of many tracks. Furthermore, the objective of an accessing servo is to achieve minimum move time between any two tracks on the disc, rather than to achieve best accuracy in following a specific track.
The transducer head control theory is to initially apply full forward power to the actuator until some point at which there is a switch to full reverse power until motion stops, hopefully at the exact center of the desired destination track. However, implementation of such a system that also has the desired reliability is prohibitively expensive, and so a slight compromise is made in control. Full forward power is initially applied, but as soon as the system velocity position corresponds to a trajectory defined by the control circuit, reverse power is applied under closed loop control. This maintains the system on the trajectory the remaining distance to the target track.
As disc drives have become more sophisticated with faster access times, the carriage, which supports and drives the transducer heads across the magnetic disc surfaces, moves faster. Consequently, there have been larger overshoots as the actual transducer head velocity approaches the trajectory defined by the control circuit, which makes the system sub-optimal.
Typically, a linear positioning motor is used as the actuator to position the magnetic recording transducer heads with respect to the magnetic disc surfaces. This type of motor includes a permanent magnet surrounding a movable armature coil which is attached to the head-positioning carriage. By passing current through the coil, forces required to move the heads from one position to another are generated. The direction of motion is determined by the polarity of the current through the coil.
As the velocity error changes polarity, the current through the movable armature coil does not instantaneously change polarity because of the inductance in the armature coils. As a result, there is a finite time after instigation of the armature current switching before the current through the armature, in fact, changes polarity. During this period of time, the current, being of the wrong polarity for proper transducer velocity control, actually aids in increasing the velocity error. This results in the velocity error overshoot being even greater, both in amplitude and duration, than overshoot from mechanical oscillations.
Larger overshoots result in longer bobbin current duration and, for short seeks, in larger current amplitude. This results in the generation of severe accelerating forces on the disc drive system resulting in longer settling time for mechanical vibrations and greater off-track problems. Both of the latter result in longer average access time.
Larger current amplitudes and longer current durations also result in greater heat dissipation and higher armature temperatures. Higher temperatures in turn create thermal off-track problems for the transducer heads. Higher temperatures also increase the effective armature coil resistance, resulting in lowering the linear motor force constant.
A few solutions have helped to decrease this problem of velocity error overshoot, but these solutions have had their own problems associated with them. For example, a higher force constant motor with low inductance can be used to decrease the velocity overshoot, but it has several disadvantages. It is very expensive to manufacture and creates greater mechanical vibrations because of the higher forces it generates. It also requires magnetic shielding to insure the safety of the recorded data on the magnetic disc pack. Also, a larger motor with higher currents requires more cooling.
Increasing bandwidth by increasing servo gain reduces overshoot and settling time and thus reduces access time, but the upper limit on bandwidth is controlled by the need to maintain stability in the presence of high frequency mechanical-structural resonances. Low pass filters can be used to filter out these noise components, but such filters introduce further lag between the desired velocity error signal and the actual velocity error signal.